1. Field of the Invention
The invention relates to a wireless communication apparatus, wireless communication method and computer program that receives a packet in conformity with a predetermined standard format and, more particularly, to a wireless communication apparatus, wireless communication method and computer program that identify and decode the format of a received packet in a network environment in which a plurality of different packet formats are mixedly present.
2. Description of the Related Art
A wireless network becomes a focus of attention as a system for freeing from wires in an existing wired communication scheme. Standards related to the wireless network may include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE802.15. For example, IEEE802.11a/g uses an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, which is one of multi-carrier schemes, as standards for wireless LAN.
In addition, the standards of IEEE802.11a/g support a modulation scheme that achieves a communication speed of 54 Mbps at the maximum; however, a next-generation wireless LAN standards that can implement further high bit rate is sought. For example, IEEE802.11n that employs an OFDM_MIMO communication scheme is defined as extended standards of IEEE802.11. Here, MIMO (Multiple Input Multiple Output) is such that a plurality of antenna elements are provided for both transmitter side and receiver side and a communication scheme that implements spatially multiplexed stream is employed. A transmission branch side spatially and temporally codes a plurality of pieces of transmission data for multiplexing, distributes the pieces of transmission data to a plurality of transmission antennas and then transmits them to a channel. On the other hand, a reception branch side spatially and temporally decodes a reception signal received by a plurality of receiving antennas via the channel, divides the reception signal into a plurality of pieces of transmission data and then acquires original data without crosstalk between streams. According to the MIMO communication scheme, it is possible to increase transmission capacity in accordance with the number of antennas without expanding a frequency band to thereby improve communication speed.
In wireless communication, generally, a preamble formed of repeated given sequences is added to the head of a packet. At the receiver side, the preamble is used to find the packet and carry out synchronization. In addition, as synchronization is completed at the preamble, control information (SIG information) described in a SIGNAL field of a subsequent header is decoded to acquire information necessary for data decoding, such as packet length, modulation scheme and encoding scheme.
A PHY layer of the above described IEEE802.11n has a high throughput (HT) transmission mode (hereinafter, also referred to as “HT mode”) of which the packet transmission mode (Modulation and Coding Scheme: MCS), such as modulation scheme and encoding scheme, is totally different from that of the existing IEEE802.11a/g, and also has an operation mode (hereinafter, also referred to as “legacy mode”) that carries out data transmission in the same packet format and the same frequency range as those of the existing IEEE802.11a/g. In addition, the HT mode may be divided into an operation mode called “Mixed Mode (MM)” compatible with an existing terminal (hereinafter also referred to as “legacy terminal”) compliant with IEEE802.11a/g and an operation mode called “Green Field (GF)” incompatible with a legacy terminal. This means that there are three types of transmission formats, that is, a Legacy Format (LF), a Mixed Format (MF) and a Green Field Format (GF). Arrangement of SIG information, content of description and degree of reliability differ among these formats. For example, an MF packet is a multiple-format packet that has multiplexed preamble information in which an HT preamble is included subsequent to a legacy preamble.
A communication terminal, when receiving a packet, identifies the format and, in addition, determines the SIG information to carry out receiving operation. The HT terminal that operates in the HT mode is able to check a received packet using the SIG information (HT-SIG) in the HT preamble having a higher check level.
For example, a wireless communication scheme has been suggested in which a certain rule is set for information described in the L-SIG of an LF packet and an MF packet, and, when irregular information is described in the L-SIG, the HT terminal discards the information read from the L-SIG as invalid data even when no parity error is detected to thereby improve false positive detection accuracy (for example, see Japanese Unexamined Patent Application Publication No. 2008-10904).
On the other hand, for frames having a higher priority, such as when an RTS (Request To Send) frame receiving station returns a CTS (Clear To Send) frame, when a data frame receiving station returns an acknowledgement (ACK) frame, and the like, an extremely short inter-frame space (Short Inter-Frame Space: SIFS) is used. Thus, strict latency restrictions are imposed on a communication terminal that receives a packet so that the communication terminal should complete decoding of SIG information within SIFS (16 microseconds) to be ready for transmission.